For some time it has been conventional practice to equip electron guns with field emission cathodes. In this arrangement, a strong electric field is applied to a sharp-pointed cathode. As a result of the ensuing tunnel effect, the field emission tip emits electrons with only weak heat or no heat applied to the cathode.
Fine-pointed cathodes permit devices to be built that afford substantially increased brightness and have smaller electron sources. This results in higher resolution than is possible with conventional thermionic cathodes.
The invention is directed to transmission and scanning electron microscopes as well as to lithographic and electron beam processing devices. It also covers devices using field ion or other thermal point cathodes.
Electron guns are made up substantially of a cathode (also referred to as an emitter), a first electrode (also referred to as a Wehnelt cylinder), and a ground electrode. In operation, an extraction voltage U.sub.s lies between the cathode and the first electrode; in field emission guns, this voltage serves to generate the electric field necessary for field emission. Accelerating voltage U.sub.b lies between the cathode and the ground anode and the acceleration path frequently includes several electrodes.
When the voltages are switched on, flashovers resulting from absorbed layers and dust particles can hardly be avoided, in particular, after a cathode has been replaced. If this occurs, the delicate points of the field emission cathodes are often destroyed. Such flashovers may occur in either the emission gap between the cathode and the first electrode or in the acceleration gap between the first electrode and the ground electrode; in the latter case, the flashover also extends to the cathode area.
U.S. Pat. No. 3,766,427 discloses an electron gun wherein the field emission cathode is protected against flashover in the emission gap by the following arrangement. The cathode is surrounded by a so-called third electrode in the form of a cylinder having a potential in the vicinity of, or equal to, the cathode potential. Within the cylinder of the third electrode at the bottom edge thereof, a so-called fourth electrode is provided to which the extraction voltage is applied. The interior of the third electrode is configured as an ion getter pump. In addition, the third electrode can be cooled using liquid nitrogen.
Electron guns equipped with arrangements to protect the cathode in the event of flashovers occurring in the acceleration path are thus far unknown.